1. Field of the Invention
The present invention relates to a wide-angle zoom lens system which is suitable for a single-lens reflex (SLR) camera, and especially suitable for a digital single-lens reflex camera.
2. Description of the Prior Art
In a digital SLR camera, the size of the imaging device is smaller than a frame size of the film for a silver-halide SLR camera. Therefore an optical system having a wider angle-of-view (shorter focal length) is necessary.
Various types of the above wide-angle zoom lens system have been known. In particular, as a wide-angle zoom lens system with a higher zoom ratio of 10 or more, the wide-angle zoom lens system of a four-lens-group arrangement, in which the positive, negative, positive and positive lens groups are provided in this order from the object, has been generally known.
The wide-angle zoom lens systems mentioned above have been designed for the image-plane size corresponding to the 135 (35 mm) film format. Therefore the maximum angle-of-view at the short focal length extremity is inevitably smaller, e.g., approximately 75°. Furthermore, in the case of a digital SLR camera having a smaller image plane (the imaging device), e.g., an APSC size image sensor, the angle-of-view at short focal length extremity is at most 55°.